Multiple melting behavior of poly (thiodiethylene terephthalate): Further investigations by means of X-ray and thermal techniques

The complex multiple melting behavior of poly(thiodiethylene terephthalate) (PSDET) was investigated by means of differential scanning calorimetry, X-ray diffraction and hot-stage optical microscopy. The phenomenon was ascribed to the existence of two different crystal structures (alpha and beta), each of them being able to melt and recrystallize under the experimental conditions adopted. Linear and non-linear treatments were applied in order to estimate the equilibrium melting temperature for alpha PSDET, by using the corrected values of T-m. The nonlinear estimation led to a higher value by about 25degreesC. Isothermal crystallization kinetics of each crystalline form were analyzed according to Avrami's treatment. In both cases, values of Avrami's exponent, n, close to 3 were obtained, in agreement with a crystallization process originating from predeterminated nuclei and characterized by three dimensional spherulitic growth. The rate of crystallization of each crystalline structure was found to become lower as the crystallization temperature is increased, as usual at low undercooling, where the crystallization process is controlled by nucleation.